You divide the damage as you cast Electrolyze, not as it resolves. Each target must be assigned at least 1 damage. In other words, as you cast Electrolyze, you choose whether to have it deal 2 damage to a single target, or deal 1 damage to each of two targets.

11/17/2017

If Electrolyze targets two creatures and one becomes an illegal target, the remaining target is dealt 1 damage, not 2. You’ll still draw a card.

11/17/2017

If all chosen targets are illegal as Electrolyze tries to resolve, it will be countered and none of its effects will happen. No damage will be dealt and you won’t draw a card.